The Cassini space probe has spotted two new Saturn moons, raising the total to 33 moons for the ringed planet, the U.S. space agency said.

The new moons could be the smallest bodies seen around Saturn, NASA said.

The moons are about 3 kilometres and 4 kilometres across, and are located between the orbits of two other Saturn moons, Mimas and Enceladus.

For now, the new moons are named S/2004 S1 and S/2004 S2. One of them may have been detected 23 years ago by the Voyager spacecraft. Back then, it was called S/1981 S14.

"One of our major objectives in returning to Saturn was to survey the entire system for new bodies," said imaging team leader Dr Carolyn Porco, from the Space Science Institute, Boulder, Colorado.

"So, it's really gratifying to know that among all the other fantastic discoveries we will make over the next four years, we can now add the confirmation of two new moons, skipping unnoticed around Saturn for billions of years until just now."

Until this latest discovery, Saturn's smallest known moons were about 20 kilometres across.

And scientists had expected that moons as small as the ones just found only to occur within gaps in Saturn's rings.

Tidal forces around the large planets cause their moons to drift away from where they were formed, NASA said.

As they drift, they may sweep by areas where other moons can disturb them, making their orbits eccentric (elliptical) or inclined relative to the planet's equator.

NASA said that one of the newly discovered moons may have gone through such a process.

The Cassini craft saw the new moons at the start of June, as it headed toward Saturn in the final stage of its seven-year journey from Earth.

Cassini, an international project involving scientists from 17 countries, has threaded its way through Saturn's rings and is now in orbit around the planet.

It is expected to spend the next four years studying the planet, its rings and its moons.